The present invention relates to a module for holding printing plates ready to be transferred onto the plate cylinders of a rotary offset printing machine. The invention also relates to a printing plate loading module for loading printing plates onto a plate cylinder of a press and to a printing press including the plate-loading module according to the invention. The invention further relates to a printing system comprising the printing plate module and a printing press according to the invention. A method of mounting printing plates onto the plate cylinder of a printing unit is also disclosed.
A web offset printing press comprises a number of printing units. Each unit is designed to print matter onto a continuous web of paper as it travels through the print unit. The printed web emerging from each print unit come together in a folding unit which orientates, folds and cuts each web to produce the finished article such as a newspaper or magazine. Each unit contains at least one pair of cylinder groups or print couples comprising a rotatably mounted plate cylinder, to which one or more printing plates are attached and, a rotatably mounted blanket cylinder. The printing unit may incorporate a shaft less drive system in which each cylinder group is driven by its own drive motor that directly drives one of the cylinders of a group via a belt or gear drive and the other cylinder of that group is mechanically coupled to the driven cylinder. An inking system associated with each print couple is operable to feed ink onto the printing plates attached to the plate cylinder as the plate cylinder rotates. As the cylindrical surfaces of the plate and blanket cylinder are in rolling contact, an inked image is transferred from the printing plates onto the blanket cylinder and from the blanket cylinder onto the medium to be printed.
In large scale high volume presses used, for example, in the production of newspapers, multi-colour printing is achieved by providing each print unit with a number of printing couples arranged in pairs which are mounted vertically above one another in a stack so that the paper web travels in a vertically upward direction between each pair of print couples. A unit having four print couple pairs, i.e. eight print couples, is able to print up to four colours on each side of the paper web and is often referred to as a “four-high” unit.
When a press is prepared for a print run, at least one printing plate carrying the image to be printed must be firmly clamped to the plate cylinder of each print couple. Generally, the plate cylinder includes at least one recess or lock-up slot extending longitudinally along the surface of the plate cylinder and into which is received the leading and trailing edge of a printing plate. A clamping mechanism located in the recess engages the leading and trailing edges of an inserted printing plate and firmly holds the printing plate in position during a print run. The number of printing plates that must be attached to the plate cylinder of each print couple depends on the width of the press and whether the plate cylinder carries one or two printing plates around its circumference, i.e. whether it is a “one around” or “two around” plate cylinder. For example, in a double width one-around press, the plate cylinder may carry four printing plates across its width and one printing plate extending circumferentially around its cylindrical surface. Therefore, there could be as many as eight printing plates in a single printing couple pair and thirty-two printing plates in a four high printing unit all of which must be replaced before a new print run can be initiated. When the plate cylinder is of the “two-around” type, the number of printing plates is doubled accordingly. Therefore, it will be appreciated that even in the production of just one newspaper issue, a very large number of printing plates will be required.
It is of utmost importance to ensure that all the printing plates attached to one plate cylinder are located in very precise circumferential and lateral registration with respect to each other. It will also be appreciated that as ink of a different colour is applied to the print medium as it passes through each print couple pair of a print unit, it is also important that the printing plates are in alignment with the printing plates attached to each of the other plate cylinders of the press as any misalignment of a printing plate will result in mis-registration of the different coloured inks applied to the print medium which will reduce the quality of the final print.
To enable accurate location of the printing plates, precise detection and confirmation of the position of each printing plate must be determined during installation onto the plate cylinder. A commonly known method of aligning a printing plate on a plate cylinder and to make sure that it is located in an “in-register” position is to provide the leading edge of each plate with a number of slots that locate on pins in the recess in the plate cylinder into which the leading edge of the printing plate is received when it is attached to a plate cylinder. The position of the slots and pins are predetermined so that, when the pins have been located in the slots, the printing plate is in the correct position and the press operator can be confident that the printing plate will be positioned correctly in relation to other printing plates.
In a conventional printing machine, the press operator loads printing plates onto the plate cylinders manually so that they are accurately located in their predetermined in-register positions. However, it will be appreciated that this activity is very labour intensive and time consuming especially when a large number of printing plates need to be replaced. It also means that the press is rendered inoperable for an extended period of time and this has a significantly detrimental affect on the overall productivity of the press. In fact, many press operators simply cannot afford to shut down a press for the amount of time it takes to complete a manual plate change due to increased demand for printed matter and the very tight deadlines that need to be met.
In an attempt to reduce the setting-up time, automatic or semi-automatic printing plate changing systems have been developed in which a printing plate is automatically fed onto, and accurately located on, the plate cylinder by a printing plate feeding mechanism attached to the printing unit. In a semi-automatic plate changing system, the press operator must accurately position the printing plates in printing plate holders fixed to the printing press adjacent to each of the plate cylinders of the press. As the position of the holder is fixed, the position of the printing plate in the holder relative to its intended position on the plate cylinder is known and the printing plate can be conveyed onto the plate cylinder with its “in-register” position maintained. In an automatic printing plate changing system, the position of the plate does not rely on the accurate positioning of the printing plates in the holder but is determined by position sensors or other devices whilst the plate is fed onto the plate cylinder or, once the leading edge of the printing plate has been inserted into the recess in the plate cylinder. Therefore, in an automatic plate changing system, the press operator does not have to concern himself with the precise positioning of the printing plates within their holders.
A disadvantage with semi-automatic plate changing systems is that the press operator still has to accurately locate the printing plates in each of the holders adjacent to each of the plate cylinders on the press itself. Although this does reduce the plating-up time, as the printing plates for the next print run can be located in their holders whilst the press is still operational with another set of printing plates, it is still undesirable for people to be working on the press whilst it is running as this can be dangerous, especially when loading the holders located adjacent to the uppermost plate cylinders which are located in an elevated position. Furthermore, sensors or other position location devices are still required to ensure that the printing plates are positioned accurately with respect to the plate cylinder onto which they are being loaded during and/or after the loading process when the plates are being conveyed from their holders onto the plate cylinder.
Although an automatic plate changing system alleviates the problems with a semi-automatic system, the operator still needs to place new printing plates into the holders ready for the next print run and so must be working on the press whilst it is still running and so the potential danger associated with this task is still not avoided or reduced. Furthermore, an automatic plate changing system requires position sensors and/or other devices to accurately determine the location of the printing plates themselves as they are conveyed onto the plate cylinder. Even when the position of the printing plates before loading begins is found to be accurate, it is often found that one or more plates may shift slightly during loading and so repeated checking of the plate positions once they have been located on the plate cylinder is required.
The present invention seeks to overcome or alleviate the aforementioned problems with manual plate loading procedures as well as those associated with semi-automatic or automatic plate loading systems thereby reducing the time taken to replace the printing plates on each plate cylinder and the printing press as a whole. It also ensures more precise positioning of the printing plates as they are loaded onto the plate cylinders thereby avoiding the need for complicated detection and position sensing equipment for detecting the position of the printing plates.